1. Field of the Invention
The present invention relates to a method and a machine for use in press-forming glass.
2. Description of the Related Art
Methods for manufacturing glass elements, such as optical lenses with high accuracy, are roughly classified into two groups: One group includes methods of grinding and polishing a glass material, whereas the other group includes methods of press-forming a glass material under elevated temperature.
The former method is employed generally to manufacture optical lenses. However, this method has problems. One is that more than ten steps are required for forming a curved surface. The other is that grinding and polishing produce a large amount of glass powder harmful to the operator. This method further has a drawback in that it is difficult to manufacture a large number of high-priced lenses having aspherical optical surface with the same accuracy.
On the other hand, the latter method employs a preform formed by cooling molten glass in a mold. The preform is heated again and subjected to press forming. In this manner, the pattern of press dies is transferred to the preform to produce an optical lens. This method is an advantage in that a curved surface is formed only in a single press-forming step. This method has another advantage: once the press dies is prepared, a large number of products can be manufactured with the same accuracy, which is determined by the accuracy of the press dies.
In the press-forming method, to increase the life of the press dies, press forming is desirably performed in an inert atmosphere or a vacuum. The press forming is performed by the following steps. First, a preform is placed between press dies. After the press-forming chamber housing the press dies and the preform is filled with an inert gas or evacuated, the press-forming chamber is heated by an infrared lamp (or a high-frequency heating unit). When the temperature of the preform reaches a predetermined value, the preform is subjected to press forming by use of the press dies. The press-formed product is thereafter cooled and taken out from the dies.
In recent years, various glass elements have been used in the optical communications field and the medical field. Of them, the optical elements made of quartz glass have been drawn attention by the reasons that they have a low thermal expansion rate, a small content of impurities, and an excellent ultraviolet light transmittance. There are a wide variety of quartz optical elements different in shape ranging from simple to complicated ones (e.g., a micro lens array) and different in size ranging from ultra micro to large ones.
A general optical lens is press-formed at a temperature at most about 700° C., whereas a quartz optical element at a higher temperature of about 1400° C. To perform press forming at such a high temperature, a high-power infrared lamp must be used.
During heating the press-forming chamber by an infrared lamp or maintaining it at a predetermined temperature, if the press-forming chamber is evacuated, gas flow is generated and deprives heat from the press dies and the peripheral members, with the result that the temperature of the press dies and a preform decrease. In particular, when a high-power infrared lamp is used, the temperature goes up and down (hereinafter, this phenomenon will be referred to as “temperature hunting”) along with ON and OFF operations of the infrared lamp, which is performed to maintain the chamber at a preset temperature. As a result, the dimensional accuracy of the formed product deteriorates.
When the press dies and preform are surrounded by an inert gas atmosphere, heat is transmitted from the inert gas atmosphere to the press die and the preform. Since the heat of the inert gas atmosphere is equalized by convection, heat can be equally transferred to the press dies and preform. In contrast, when heating is performed after the press-forming chamber is evacuated, only the heat radiated from the infrared lamp is transmitted. In the case of a transparent perform, infrared rays pass through the preform and do not heat the preform, directly. Therefore, the preform is indirectly heated through heat transmission from the press dies.
A preform frequently used for forming a lens is in contact with the press dies at a small area. Since heat is transmitted only through the small area, a big difference in temperature is produced between the preform and the press dies. Since heat is supplied to the press dies from the outside by an infrared lamp, part of the press dies placed in the shadow of the press dies themselves is not irradiated with the infrared rays. As a result, when heating is performed only by radiation, the temperature difference easily occurs.
As described previously, to prevent oxidation of the press dies, the air of the press-forming chamber is purged and filled with an inert gas. However, it is difficult to completely purge the air retaining in small and thin portions of the die surfaces. It is therefore not easy to prevent oxidation of the press dies. Furthermore, when the press dies are placed in an inert gas atmosphere, a pattern of the press dies is not faithfully and accurately transferred to a preform. To improve the accuracy of the pattern transfer and prevent the oxidation of the press dies, reducing the pressure of the press-forming chamber is helpful rather than filling the chamber with an inert gas. Accordingly, a method performed in a low-pressure press-forming chamber is widely employed.
However, thermal behavior in the press-forming chamber differs between at normal pressure and reduced pressure. Even if a predetermined temperature is maintained by feedback control at normal pressure, inert gas flow is generated by evacuation of the press-forming chamber, changing temperature. Therefore, it takes time to stabilize the temperature. In some cases, the hunting of temperature is never settled. If the temperature of the press-forming chamber is not stabilized, it is difficult to obtain a high-quality press-formed product. In particular, when the hunting increases and temperature rises excessively high, the glass of a preform is thermally decomposed. As a result, the transparency of the glass decreases, failing to satisfy the specification for an optical element.
In addition, there is another problem: the position of the preform shifts on the press dies by gas flow when evacuation is initiated. Consequently, the dimensional accuracy of the press-formed product decreases.